Conventionally, engines have been used as the sole power source of an automobile. In recent years, a metal wheel motor has attracted attention as an automobile's power source, replacing the engine.
The metal wheel motor has benefits including high power transmission and highly responsive control, which can be conducted independently for each wheel.
In addition, installation of a metal wheel motor on each of the four wheels eliminates the conventional, complicated driving system (transmission, drive shaft, differential gear, and the like), enhancing the degree of freedom in under-floor layout.
Since recently, various proposals have been made for metal wheel motors having the various advantages mentioned above.
For example, an outer rotor-type metal wheel motor is known which comprises a disc metal wheel having a rim part, a disc part and a motor stator fixed to a hub-supporting part, in which at least one of the rim part and the disc part constitutes a motor rotor and the motor rotor has the same structure as an induction motor (see Patent Document 1, for example).
As an example of other structures, a switched reluctance motor is known which comprises a motor rotor provided with a plurality of rotor poles fastened to the inner periphery of the rim part, projected in the direction of a supporting shaft, and are arranged along the circumference, and a motor stator provided with a plurality of stator poles fastened to the supporting shaft, projected in the direction away from the supporting shaft and are arranged along the circumference (see Patent Document 2, for example).
As yet another example of other structures, a high-rotary speed, electrically-driven wheel is known which comprises several electromagnets arranged on the non-rotating part of a wheel in plural circular rows integrally or circumferentially, and a rotary shaft formed of a non-magnetic material on which a plurality of magnets are provided on one or both sides of the electromagnets at a position opposite to the electromagnets in such a manner that the magnets are arranged at an equal interval along the circumference and are nearly in contact with the electromagnets with a small slit therebetween, wherein the magnetic polarity and position of the rotating magnets are detected by a magnetic sensor. A semiconductor for switching electrical current is actuated to switch the positive and the negative electrical currents to change the polarity of the electromagnets, thereby causing driving power to generate. The discontinuation period of the electrical current and the amount of electric power are controlled, whereby the control of the rotation speed of the wheel is realized (see Patent Document 3, for example).
These are outer rotor type metal wheel motors in which a motor rotor is provided on the outside of the motor stator.
In such outer rotor type metal wheel motors, output and torque can be increased easily. Furthermore, a loss in driving power can be reduced since a reduction gear is not necessary, and an increase in unsprung weight can be suppressed. In addition, due to the elimination of a reduction gear, a motor is easily installed in a metal wheel house. Furthermore, due to the hollow inside structure, it is possible to accommodate other components such as a brake.
In addition, due to these structural advantages, a motor can be provided to front wheels having a steering mechanism, realizing a four-wheel metal wheel motor automobile.    Patent Document 1: JP-A-10-305735    Patent Document 2: JP-A-2004-343905    Patent Document 3: JP-A-07-101252